1. Field of the Invention
Embodiments of the present invention relate to an image forming apparatus including a recording head that forms an image by discharging the same color of ink from plural nozzle sequences, an image correction method, and an image correction program.
2. Description of the Related Art
A configuration has conventionally been known where a recording head is mounted on an inkjet printer. Here, the recording head includes plural nozzles from which ink is discharged. In such an inkjet printer, pressure is applied to ink liquid chambers disposed in the recording head by using a piezoelectric element or a heater element, and the ink is discharged toward a recording medium. Thereby, an image is formed.
Examples of the conventional inkjet printers include a serial inkjet printer that forms an image by moving the head in a direction that is perpendicular to a sheet conveyance direction and a line inkjet printer that forms an image by fixedly arranging plural heads in line and by conveying a sheet of paper in a direction that is perpendicular to a longitudinal direction of the heads. For both the serial inkjet printer and the line inkjet printer, the image quality depends on the adhering property of ink on a sheet of paper. Therefore, techniques for adjusting the discharging property and the discharging timing of the recording head have been developed. For example, Patent Document 1 (Japanese Patent Laid-Open Application No. 2001-105635) discloses a technique for aligning print positions among plural print heads. Patent Document 2 (Japanese Patent Laid-Open Application No. 2008-162067) discloses a technique for preventing positional shifts of positions where dots are recorded. Patent Document 3 (Japanese Patent Laid-Open Application No. H5-124221) discloses techniques for adjusting registration and correcting density unevenness.
Incidentally, conventional recording heads include recording heads having plural nozzle sequences. Examples of the recording head having the plural nozzle sequences include recording heads in which the nozzle sequences are arranged while the nozzle sequences are shifted from each other and recording heads in which the nozzle sequences are arranged in parallel. When the nozzle sequences are arranged while the nozzle sequences are shifted from each other, the resolution in the longitudinal direction of the heads is improved. When the nozzle sequences are arranged in parallel, the resolution in the discharging direction is improved. Further, when the plural nozzle sequences are arranged in parallel, even if there is a nozzle that fails to discharge the ink, the failure on the image can be made unnoticeable or the image can be interpolated by another nozzle.
In the conventional recording head including the plural nozzle sequences, an ink discharging characteristic of a nozzle sequence may be different from that of another nozzle sequence. The difference of the discharging characteristic may be attributable to many reasons, such as manufacturing variations of the components included in the nozzle sequences and the circuits for driving the nozzle sequences, and the differences among the flow characteristics of the ink flowing through the corresponding nozzle sequences due to the positional differences of the corresponding nozzle sequences. When there are differences among the discharging characteristics of the corresponding nozzle sequences, the adhering conditions of the ink droplets to the recording medium may vary. Therefore, such differences may cause an unintentional failure on the image.
The discharging characteristic of the nozzle sequences may be corrected by correcting signals that are applied to the nozzle sequences. However, for the case of the recording head having the plural nozzle sequences, since the discharging characteristic of a nozzle sequence may be different from that of another nozzle sequence, a sufficient correction effect may not be obtained only by correcting a signal that is commonly applied to the nozzle sequences.
Further, when the recording head includes the plural nozzle sequences, an image is formed by combining all the nozzle sequences. Therefore, an optimum adjustment value for adjusting the discharging characteristic for a case where the ink is individually discharged from the corresponding nozzle sequences may be different from that of another case where the ink is discharged from all the nozzle sequences.
Hereinafter, there will be explained density adjustment processes of a recording head including nozzle sequences 11 and 12 during an image formation process, while referring to FIGS. 1 and 2. FIGS. 1A and 1B are first diagrams illustrating the density adjustment processes of a conventional recording head including two nozzle sequences during the image formation process. FIG. 2 is a second diagram illustrating the density adjustment process of the conventional recording head including two nozzle sequences during the image formation process.
In FIG. 1A, a density value of an image formed by the nozzle sequence 11 and another density value of another image that is separately formed by the nozzle sequence 12 are within the corresponding target ranges. Additionally, in FIG. 1A, a density value of an image formed by combining the nozzle sequences 11 and 12 is within a target range. In FIG. 1B, additional separated lines are observed in an image formed by the nozzle sequence 12. Here, the density values of images individually formed by the nozzle sequence 11 and the nozzle sequence 12 are in the corresponding target ranges. However, when an image is formed by combining the nozzle sequences 11 and 12, gaps are observed in the formed image, and the density value of the image does not reach the target range.
In such a case, the density value of the image may be increased by increasing driving voltages of the nozzle sequences 11 and 12. However, when the driving voltages of the nozzle sequences 11 and 12 are uniformly increased, the stability of the discharging characteristic of the nozzle sequence 12, whose ink discharging characteristic has been unstable, may become worse.
Further, in FIG. 2, the density value of the image formed by combining the nozzle sequences 11 and 12 is within the target range. However, when the nozzle sequences 11 and 12 are individually observed, it can be found that an ink discharging amount of the nozzle sequence 11 is large and that an ink discharging amount of the nozzle sequence 12 is small. Therefore, when an image of evenly spaced lines are formed by the recording head, an image failure may occur, since a width of each of the lines formed by the nozzle sequence 11 and a width of each of the lines formed by the nozzle sequence 12 are different.
Further, when a driving condition of the nozzle sequences are changed, it is possible that positions on the sheet of paper where the ink discharged from the nozzle sequences are adhered are varied. Further, even if the discharging characteristics of the plural nozzle sequences are the same, a failure may occur in a formed image when the plural nozzle sequences are combined. FIGS. 3A and 3B are diagrams illustrating cases where corresponding images are formed by combining plural nozzle sequences having the same discharging characteristics.
In FIGS. 3A and 3B, the ink discharging characteristics of the nozzle sequences 11 and 12 are the same. In FIG. 3A, relative adhering positions of the ink discharged from the nozzle sequences 11 and 12 are in accordance with the target positions. On the other hand, in FIG. 3B, adhering positions of the ink discharged from the nozzle sequence 11 are relatively shifted from the corresponding adhering positions of the ink discharged from the nozzle sequence 12. As described, even if the plural nozzle sequences having the same ink discharging characteristics are combined, a manner of covering a recording medium with the ink may be different from a usual manner, and a failure may occur in the formed image.
The embodiments of the present invention have been developed in view of the above circumstances. An objective of the embodiments of the present invention is to provide an image forming device, an image correction method, and an image correction program, with which plural nozzle sequences can be optimally adjusted.